Using GWLF for Development of “Reference Watershed Approach” TMDLs
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Total maximum daily loads (TMDLs) are required to be developed in each state for stream
segments that have been listed on their 303(d) list of impaired streams. Where a numeric standard
has been set for a pollutant, the indicator of a restored condition – the TMDL target load – is readily
definable. Where a quantitative standard does not exist for a pollutant, as is the case for sediment
and nutrients in many states, an alternative approach is required to define the TMDL target load.
One such alternative is the Reference Watershed Approach. This paper describes how the GWLF
model has been used with the Reference Watershed Approach to develop TMDLs for benthic
impairments in Virginia based on long-term average annual sediment loads. Although developed for
use with ungaged watersheds, studies have shown that calibration improves modeled stream flow,
where observed data is available. A modification of the GWLF code is, therefore, described that
allows output of monthly pollutant loads by land use to enable calibration to monthly model output.
This paper further describes post-processing spreadsheet procedures developed to account for point
source loading, modeling of sub-watersheds, and reductions due to implemented BMPs.
Quantification of the TMDL target load and creation of alternative allocation scenarios required as
part of initial TMDL studies are also part of the post-processing procedures.